Parcel Theory

Visualizing Atmospheric Thermodynamics on Soundings

In this first portion of the course, you are learning about the way meteorologists visualize the thermodynamic and moisture structure of the atmospheric environment. The temperature and moisture characteristics of the atmospheric environment are obtained by direct measurement of devices called "sondes" attached to weather balloons. These sondes then radio the information back to the earth where it can be plotted on what are known as thermodynamic diagrams. Such a plot is called a sounding.

Soundings would be useful if they merely showed the temperature and moisture characteristics of the quiescent or horizontally moving atmosphere. But it turns out that they are much more useful than that.

Thunderstorms, for example, are associated with cumulonimbus clouds. Cumulonimbus clouds represent the end state of the clouds of vertical development family, that has its inception with a small cumulus cloud. The small cumulus cloud can ultimately be traced from its formation, when a very small bubble of air begins to ascend relative to its surroundings.

The Ideal Gas Law and Fluid Parcels

To understand this, we assume that this one bubble, or air parcel, retains its shape and general chraracteristics as it moves up (or down) relative to the surrounding atmospheric environment. It turns out that when air parcels do this, they actually can become warmer (or colder) than the surrounding air at the same pressure elevation.

Straightforward application of the Ideal Gas Law (Equation of State) shows that such air parcels will then be less dense (or more dense) than the surrounding air at the same elevation and will spontaneously rise (or sink). The SPONTANEOUSLY rising air parcels form the updrafts in thunderstorms.

Parcel theory has its limitations, since it is difficult to define how large or small a parcel really is and what mechanisms exist that selectively lift the small parcel and not the entire atmospheric layer. In addition, parcel theory fails to accomodate the fact that considerable entrainment of surrounding air occurs as buoyant plumes develop. However, parcel theory has been shown to be successful in explaining thunderstorm development. Hence, we will use it in this presentation.

Difficulties in Conceptualization

Despite the simplicity of parcel theory, applying it in practice is often very difficult for students. This is partially because soundings apparently appear to be plots along a vertical and horizontal geographic axis (say, z and x). They are not, of course.

Of more significance is that meteorologists plot several things on such diagrams at the same time. For example, the temperature change in the vertical through the undisturbed atmospheric environment (called the Environmental Lapse Rate or ELR) is often compared to the so-called ascent curve of air parcels moving relative to that environment either adiabatically or psuedoadiabatically. Thus, the ELR, dry adiabatic lapse rate and moist adiabatic lapse rates are drawn as lines on the same diagram, yet have very different interpretations. We'll explore this by using real world illustrations, in particular, building to sthe soundings for the Joplin tornado event.